Application of an hypo-osmotic shock to isolated axons of Carcinus maenas induces a decrease in the intracellular content of K+, Na+ and Cl−. The changes in Na+ and Cl− levels are only transitory while the K+ level reaches new steady-state value much lower than the control. The modification of K+ concentration cannot be ascribed only to a simple dilution process and it is proposed that the regulation of intracellular K+ plays an important role in limiting the swelling which occurs in this tissue.Hypo-osmotic conditions also induce an increase in Na+ permeability.The results are discussed in relation to changes in ion transport and interactions with intracellular organic compounds that could arise during the process of volume regulation.